1. Field of the Invention
The present invention relates to digital communications systems in which the carrier signal is varied in accordance with bursts of binary data being transmitted. More particularly, the present invention pertains to a burst mode data communications technique in which the carrier signal is optimized for bit synchronization and immunity to impairments arising from signal multi-path, in wireless applications, and reflections, due to impedance discontinuities in wire line applications.
2. Description of the Related Art
Digital modulation is the means by which a carrier, inherently, analog in nature, is made to carry digital information in a communications channel. This involves altering the amplitude of the carrier, the angular velocity of the carrier, or both. The objective is to create discrete phase/amplitude states in a manner that leaves little chance for ambiguity between the states. These discrete states, commonly designated as symbol states, correspond, to one or more binary bits of data. By regenerating the binary data at intervals along a transmission path, re-transmitting data that is determined to be corrupt and applying error correction, digital data can be transmitted over great distances with no degradation of quality, even in the presence of a significant level of background noise. This is in stark contrast to analog modulation in which noise effects accumulate through the transmission path, usually without remedy.
In digital communications channels, noise is only one problem which must be overcome. In wire line applications impairments in cables cause impedance discontinuities, whereby resultant distortion of the signal and recovery of the digital data at the receiver becomes much more difficult. In wireless applications, multi-path is a degrading factor, especially with antennas of low, directivity. The transmitted signal traverses the most direct and desirable path to the receiving antenna. However, the signal may traverse an indirect path as well, being reflected from a buildings or other object, and also arrive at the receiving antenna. Multi-path causes a summation of such reflected signals, delayed in time, with the main signal. This also makes recovery of the digital data more difficult.
The prior art uses techniques such as training sequences, which allow equalization to be gradually varied until the equalization settings are optimized. This approach works well with continuous data streams that last for seconds or more, but does not work very well for burst mode transmissions, which may originate from multiple points. The transmitter in this case bursts out data for short periods of time and then becomes quiet to allow other transmitters a chance to send data. For efficient use of the channel, each burst of data normally contains a preamble or header which is used for synchronization and for setting the receiver voltage thresholds. Then the payload data is sent followed by a trailer to gracefully end the transmission.
When channel impairments are present, the preamble is not always lengthy enough to allow an equalizer to train effectively. Each transmitter has its own unique path to a receiver, and associated impairments, which means the equalizer must be prepared to train on every burst or packet sequence of data. If additional header data and training time is allowed for data recovery, the effective data throughput can go down considerably. Some training sequences last for more than a second, which could translate into millions of throw away header bits at the beginning of every data burst. This is clearly undesirable when the goal is to get the maximum utilization of a communications channel.
Carrier signals for digital modulation are typically based on sinusoidal waveforms because such waveforms require the least amount of bandwidth. There exists three classical forms of digital sinusoidal modulation: amplitude shift keying (ASK), frequency shift keying (FSK) , and phase shift keying (PSK). Improvements have been made in digital sinusoidal modulation, however all of the improvements have been based on the three classical techniques previously mentioned. In ASK, the amplitude of the carrier signal is varied or shifted in response to changes in the digital data. In FSK, the frequency of the carrier signal is varied or shifted in response to changes in the digital data. In PSK, the phase of the carrier signal is varied or shifted in response to changes in the digital data.
There are also certain disadvantages associated with the classical modulation techniques. For example, ASK is especially susceptible to atmospheric noise and fading. FSK requires that an associated receiver detect two discrete frequencies before the frequency can be acquired and detected. This presents delays due to the additional time required to receive the several cycles of each frequency. PSK requires complex receiver circuitry in order to detect phase changes. Furthermore, elaborate filtering is necessary to control spurious outputs resulting from the discontinuities associated with the phase changes.
One disadvantage, however, is common to all of the classical modulation techniques and their derivative improvements. This is the use of fixed time slots for varying the characteristics of the carrier signal. When fixed time slots are used, variations in the carrier signal occur at random points along the sinusoidal waveform, thus resulting in spurious frequencies and expanding of the modulation bandwidth. Complex filtering becomes necessary in order to reduce the amplitude of these spurious frequencies. As the bit rate increases, the variations in the carrier signal occur more frequently, thus posing a challenging demodulation task.
It is well known that the amount of spurious output generated by the variation of a sinusoidal waveform is dependant upon the instantaneous value of the slope of the waveform when the changer occurs. Thus, a change which occurs at exactly the midpoint or highest kinetic energy point of the waveform generates the greatest amount of spurious output because the slope value is at its maximum. If the change occurs at exactly the peak of the waveform however, the least amount of spurious output is generated because the slope value is at its minimum, i.e., zero kinetic energy.
Representation of discrete states of digital data is accomplished through various base band encoding techniques. FIG. 2A illustrates various known base band encoding techniques. In non-return to zero level (NRZ-Level) encoding 28, digital code is produced by instantaneously shifting voltage levels at fixed bit time intervals so that two unique binary symbols are represented, a one and a zero. Thus, a one is represented by one level, while a zero is represented by the other level. The NRZ-Mark digital code 30 is produced by instantaneously shifting voltage levels at fixed bit intervals only when a one is transmitted and not changing levels when a zero is transmitted. The RZ digital code 32 is produced by instantaneously shifting voltage levels at half bit-time intervals when a one is transmitted and not changing levels when a zero is transmitted. A BI-PHASE-Level digital code 34 is produced by instantaneously shifting voltage levels at half bit-time intervals so that a one is a high level during the first half of the bit time and a zero is a high level during the second half of the bit time. The NRZ-4Level digital code 36 is produced by shifting voltage levels instantaneously after two, bit-time intervals. A one--one is transmitted by the top level, a one-zero is transmitted by the next lower level, a zero-one is transmitted by the next lower level, and a zero-zero is transmitted by the bottom level. By encoding two bit times into one symbol, NRZ-4Level encoding reduces the effective transitioning rate in half. Similarly, three bit times could be encoded into NRZ-8Level encoding to reduce the transitioning rate by one third. Multi-level encoding suffers from a drawback in that it necessitates a more complex receiver to detect and recover the transmitted symbol.
The prior art creates transitions at fixed bit times when encoding digital symbols. These transitions occur at the bit edges and remain steady until the next bit edge, thus requiring the transmission of a step function or rectangular pulse. The rectangular pulse or step function generates spurious energy components at frequencies from zero to infinity. Consequently, frequency multiplexing of data over a communications channel cannot be performed unless the instantaneous voltage transitions of the rectangular pulse are filtered to remove unwanted energy components throughout the frequency spectrum. The rectangular pulse and its corresponding bandwidth are shown in FIG. 3.
As an example, consider data transmitted at 1000 bits/second. The bit time T would be {fraction (1/1000)} of a second, or 1 millisecond. The first frequency null would occur at 1/T or 1000 Hz and energy components would extend from zero hertz to infinity. When the rectangular pulse is used to modulate a sinusoidal carrier, the negative frequency spectrum would also be translated up in frequency and the null to null bandwidth would be 2/T or 2000 Hz. However, energy outside of the main lobe is not required to recover the transmitted data. Furthermore, it is desirable to reduce the modulation bandwidth by eliminating frequency energy outside this primary lobe.
In order to transmit the maximum amount of data over a given digital communications channel, a small bandwidth and high bit rates are required. For example, more data can be transmitted over a frequency multiplexed communications channel, such as those used in satellite links, when data communication techniques are used that have narrow bandwidth requirements per carrier signal whiled still providing high data rates.
Much development work has gone into ways of minimizing the bandwidth of the rectangular pulse. In the prior art, the instantaneous transitions at the bit edges are smoothed by passing the rectangular pulse through filters. While the filtering process removes many of the unwanted frequencies outside of the main modulation lobe, it also removes some of the energy which helps in the symbol recovery process. Thus, a delicate balancing act must be made in choosing a filter suitable for reducing the bandwidth while not seriously degrading the modulation information contained within the symbol. It has been shown by Nyquist and others that the band limited rectangular pulse has an ideal shape after filtering which is called the sin x/x or sinc pulse.
A filtered rectangular pulse which closely approximates the ideal sinc pulse shape is commonly used to modulate a high frequency sinusoidal carrier, thus producing an upconverted frequency multiplexed communications channel which can transmit numerous messages simultaneously. Each digitally encoded base band signal is subsequently modulated with a carefully chosen local oscillator frequency in order to pack the greatest number of carrier signals within a given communications channel.
Recovery of individual carrier signals is accomplished through the use of a carefully created local oscillator capable of downconverting the carrier signal to the original digitally encoded base band signal. Due to the pulse filtering during the modulation process, the received base band signal has an xe2x80x9ceye patternxe2x80x9d associated with it. This eye pattern is the result of the vertical edges of the rectangular pulse being filtered and the shape of the xe2x80x9ceyexe2x80x9d opening is based on the sinc pulse shape.
Conventional demodulation of the data in the two symbol case is accomplished by using a zero voltage reference. A measurement is made at the center of the bit time in order to determine whether the received symbol is above or below this zero voltage level. It is important to base the decision of which bit was received precisely at the midpoint of the bit time because the least amount of ambiguity exists between symbols at the midpoint of the bit time. Furthermore, the decision of which symbol is received is complicated by interference and noise that is present to some degree in all communications channels. The interference and noise causes the eye opening to close, thus making it critical to choose the optimum point for symbol decision making.
Further reductions in the bandwidth can be gained when more than two symbols are utilized. The number of symbols increases as a power of 2. For example, 2, 4, 8, 16, 32, 64, 128, 256, 512, and 1024 symbols are used in modern communications systems. To achieve a large number of unique and distinctive symbols, changes in carrier phase and amplitude are typically used. To go from 2 to 4 symbols gives a reduction of 2 in bandwidth. Eight symbols reduce bandwidth by one third, sixteen symbols reduces bandwidth by one fourth and so on. The penalty for increasing the number of symbols is the increased difficulty in determining which symbol was actually received.
When the binary case of two symbols is used, there is only one voltage reference needed to determine if the eye pattern is above or below the reference. However, when many symbols are used to reduce bandwidth, a corresponding increase in symbol amplitude and phase references are needed. This is further complicated by thee fact that interference and noise are always present in the channel, resulting in a greater possibility of making an incorrect symbol decision when an increased number of symbols are used.
The related art is represented by the following patents of interest.
U.S. Pat. No. 3,713,136, issued on Jan. 23, 1973 to John Nagy, Jr., describes an analog-to-digital converter of the dual slope integrating type that operates without counter reset or input gating circuitry to provide successive, accurate digital readouts representing the average magnitude of corresponding analog input signals. Nagy, Jr. does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 3,980,826, issued on Sep. 14, 1976 to Albert X. Widmer, describes a bifrequency encoded binary data transmission system wherein the transmitted signal is predistorted to eliminate the need for equalization at the receiving end. Widmer does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 4,395,642, issued on Jul. 26, 1983 to Stefan Traub, describes a sine-shaping circuit which is capable of generating a sinusoidal signal having low harmonic distortion, continuous slope and high accuracy. Traub does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 4,435,824, issued on Mar. 6, 1984 to Brian W. Dellande et al., describes a communications system which generates an output signal of a selected primary frequency having digital data bits serially modulated thereon. The system utilizes a hybrid differential phase-shift keyed (DPSK) modulation. The improved DPSK modulation is frequency modulation of a phase encoded signal. A controller is used to control a frequency generator means which is used to shift the frequency of the output signal between primary and secondary frequencies. The controller and frequency generator means provide a modulated output signal which is D.C. balanced bit by bit and has substantially reduced harmonic energy. Dellande et al. do not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 4,472,747, issued on Sep. 18, 1984 to David M. Schwartz, describes a microcomputer system for converting an analog input signal into a digital form for storing in digital form in a highly condensed code and for reconstructing the analog signal from the coded digital form. Schwartz does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 4,564,823, issued on Jan. 14, 1986 to John P. Stahler, describes a modulation system wherein a carrier signal is modulated by an input signal having a variable amplitude. The amplitude of the input signal is sampled every predetermined fractional cycle of the modulated signal. The carrier signal is modulated in response to the sampled amplitude of the input signal. This provides a modulated signal having an amplitude and duration which are inversely proportional to each other, but related to the sampled amplitude of the input signal, for each predetermined fractional cycle. The signal is subsequently demodulated by zero-crossing detection, peak-amplitude detection, or a combination of both. The system is self-clocking, does not produce discontinuous phase or amplitude changes, and does not introduce DC components. Stahler does not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 4,596,022, issued on Jun. 17, 1986 to Donald L. Stoner, describes a system for communicating digital data over a limited bandwidth transmission link. The system utilizes modulator means for receiving an input digital signal and generating a corresponding frequency shift keyed signal having high-frequency and low-frequency signals with in-phase signal transitions at frequency shifts. Demodulator means are provided for receiving-and demodulating the frequency shift signal in order to generate an output digital signal corresponding to the input digital signal. The demodulator means includes a zero crossing detector in order to recover the frequency shift keyed signal received. The system includes a transition detector for detecting transitions in the binary logic state on an input digital signal and generating a transition indicator signal in response thereto. A frequency shift key having an oscillator capable of generating high and low frequency signals is used to provide substantially in-phase signal shifts. Stoner does not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 4,745,628, issued on May 17, 1988 to William T. McDavid et al., describes a symbol generator for phase. modulated systems. The symbol modulator produces a filtered analog waveform for use in phase modulating a carrier. The symbol generator includes a memory for storing digital representations of analog waveform segments at predetermined addressable locations. Each segment corresponds to the cross-correlation of a predetermined filter function with a predetermined number of data bits in the data stream. The data stream is converted to an address for the memory to output a digital value to a digital to analog converter. The output of the digital to analog converter includes an in-phase and quadrature-phase components which are directed to a sample and hold circuit for generating the analog signals. The waveform segments are then sequentially assembled and directed to a vector modulator. McDavid et al. do not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 4,809,205, issued on Feb. 28, 1989 to Richard A. Freeman, describes a direct digital synthesizer which digitally generates a sinusoidal waveform by dividing the sine waves into a plurality of coarse phase angle intervals, which are in turn divided into a plurality of intermediate phase angle intervals. Freeman does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 4,871,987, issued on Oct. 3, 1989 to Seiji Kawase, describes a binary signal modulator having a circuitry for sampling a binary signal at a predetermined sampling frequency. A modulating circuit responsive to the sampled binary signal is used for generating a modulated signal whose rising and decaying timings are respectively determined by the start end times of the binary signal. The rising and decaying timings of the modulated signal are defined as predetermined functions. Kawase does not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 4,897,620, issued on Jan. 30, 1990 to Ronald Y. Paradise, describes a continuous phase shift modulation system with improved spectrum control. The inventive method includes the step of determining, in advance, whether successive pulses are to have the same or different polarities for each of the in-phase and quadrature components. If successive components are to have the same polarity, then a continuous transition modulation signal between the successive pulses is provided in place of adjacent portions of the successive half-cosine pulses. The modulation of the other component is adjusted during the time of the continuous modulation signal so as to maintain a desirable constant amplitude characteristic. Paradise does not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 4,905,177, issued on Feb. 27, 1990 to Lindsay A. Weaver, Jr. et al., describes a method and apparatus for converting phase data into amplitude data. Weaver, Jr. et al. do not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 4,975,699, issued on Dec. 4, 1990 to Gary D. Frey, describes a circuit for generating an analog sine voltage from a digital phase input employing a memory storing sine and cosine values and a correction value for each phase and first and second digital-to-analog converters. Frey does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 4,995,059, issued on Feb. 19, 1991 to Hisashi Ishikawa, describes a predictive coding device with an input terminal through which a sampled value is input, a predicting circuit for outputting a predicted value for the sampled value which has been input through the input terminal, and a look-up table arranged to receive, as a readout address, the sampled value input through the input terminal and the predicted value and then to output a predictive coded value. Ishikawa does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,012,491, issued on Apr. 30, 1991 to Motoya Iwasaki, describes a burst mode digital communications system with a preamble detection circuit that requires a small amount of preamble information for establishing carrier and clock timing and controlling preamplifier gain by deriving error control signals simultaneously during the reception of a preamble. Iwasaki does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,046,071, issued on Sep. 3, 1991 to Toshiyuki Tanoi, describes an encoding device for transforming a sequence of digital image signals into a sequence of transformed signals which is subjected to interframe predictive coding by the use of a subtractor and a local decoding loop. Tanoi does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,237,324, issued on Aug. 17, 1993 to Alfredo R. Linz et al., describes a system for producing al modulation baseband analog signal responsive to serial bits of digital data. Linz et al. do not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,357,544, issued on Oct. 18, 1994 to Thomas G. Horner et al., describes a method of decoding a composite signal which includes receiving a composite signal including a pilot signal at a pilot signal frequency and first and second information modulated with a subcarrier at a harmonic of the pilot frequency. Horner et al. do not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,361,046, issued on Nov. 1, 1994 to John D. Kaewell, Jr. et al., describes a modulator capable of providing a fractional sample or symbol time. The modulator employs a decimation counter responsive to a clock having a frequency of M/N*symbol clock, where M is an interpolation factor and N is a decimation factor. The modulator uses this frequency to generate a data symbol clock to select frequency shift key (FSK) symbols from a sampled data array. A multiplier receives and multiplies the FSK symbols by a weighing factor which is determined by the decimation counter. The modulator allows digital modulations which consist of a non-integer number of samples per symbol time to be synthesized in an efficient manner. The modulator is also capable of producing fractional sample and symbol modulations in order to allow support of modulations with various symbol rates by hardware platforms which contain fixed digital to analog sampling clocks. Kaewell, Jr. et al. do not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 5,406,584, issued on Apr. 11, 1995 to David Erisman, describes a time shift keying digital communications system. The system utilizes a digital modulation technique which is unique in that fixed time slots are not used to vary the characteristics of the carrier signal. Instead, variations in the time slots are used to transfer the digital information. The modulation is created by synthesizing a carrier waveform capable of varying the time it takes for each peak to occur. The peaks of the carrier signal are tightly controlled to occur at exact discrete time slots corresponding to the base band digital signal. Erisman does not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 5,430,764, issued on Jul. 4, 1995 to William A. Chren, Jr., describes a direct digital frequency synthesizer that employs residue number system based processors to generate output waveforms of desired frequencies. Chren, Jr. does not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,467,294, issued on Nov. 14, 1995 to Vince Hu et al., describes a method and apparatus suitable for generating programmable digital sine waves which involves converting the output of a direct digital synthesizer or numerically controlled digital oscillator to a higher frequency with a multiplier-less structure that takes advantage of the. properties of trigonometric identifiers for sine and cosine. Hu et al. do not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,479,168, issued on Dec. 26, 1995 to Keith O. Johnson et al., describes digital encoding/decoding methods and apparatus for ultra low distortion reproduction of analog signals which are also compatible with industry standardized signal playback apparatus. Johnson et al. do not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,479,451, issued on Dec. 26, 1995 to Charles Eldering et al., describes a method for data recovery in burst mode communications systems that include a data signal having a burst preamble with a defined sequence of m bits, by sampling in a first sampling step the burst preamble with n samples per bit carried out by n bit clocks with different phases, and providing a sampled burst preamble. Eldering et al. do not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,481,230, issued on Jan. 2, 1996 to Paul Chang et al., describes a phase modulator circuit and a method for generating an output signal having individually positionable edges. The phase modulator includes a programmable pulse generator for producing an output signal and a control value source for delivering a sequence of control values to the generator. The control values determine the time between successive output pulses. Succeeding control values are provided in response to the edges of the output signal and each next control value in the sequence is made available to the programmable pulse generator within the time between successive edges of the output. Chang et al. do not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 5,513,219, issued on Apr. 30, 1996 to Ronald E. Ham, describes a method and apparatus for transmitting information at a high rate by using an undermodulated frequency shift keyed signal. The transmission rate is independent of the data content and the system requires no zero crossing detectors. The apparatus also includes a demodulator which combines non-linear processing circuitry with a conventional demodulator. Ham does not suggest a burst mode digital communications system according to the claimed invention.
U.S. Pat. No. 5,621,766, issued on Apr. 15, 1997 to Bradley B. Bakke et al., describes a signal receiver using a burst detector to detect the occurrence of a burst. Bakke et al. do not suggest a burst mode digital communications system in accordance with the claimed invention.
U.S. Pat. No. 5,642,386, issued on Jun. 24, 1997 to A. Gregory Rocco, Jr., describes an all-digital data sampling circuit and related sampling method for a burst mode data communications system. Rocco, Jr. does not suggest a burst mode digital communications system in accordance with the claimed invention.
None of the above inventions and patents, taken either singularly or in combination, is seen to describe the instant invention as claimed.
The present invention is a burst mode digital communications system which provides a method for digitally modulating a sinusoidal carrier. The method utilizes direct digital synthesis techniques to create discrete carrier segments having zero slope edges. The discrete carrier segments may contain different time, phase, or amplitude values and can be combined with other discrete carrier segments having zero slope edges or zero slope level segments to modulate the sinusoidal carrier. A table of phase amplitude values is first created using direct digital synthesis, techniques and subsequently stored in a read only memory (ROM) look-up table. A plurality of digital data blocks are then sent to an associated transmitter and modulated. The modulation is performed such that the bit edges of each of the digital data blocks occur at zero slope points where no voltage step changes take place. The stored phase amplitude values for each digital data block are then passed through a digital-to-analog converter, thereby producing an analog waveform. A microprocessor and a phase accumulator may be utilized to accurately control reception and, modulation of the digital data blocks. In preferred embodiments off the invention, the output of the digital-to-analog converter may be filtered in order to remove sampling artifacts and produce a low distortion waveform.
The present invention also provides a method for demodulating data which has been modulated by discrete carrier segments having zero slope edges. The data received is in the form of a carrier signal which is composed of carrier segments. The carrier segments are filtered in order to remove unwanted noise. The carrier segments of the modulated data may then be recovered by detecting the presence or absence of the carrier segments within a predetermined bit time or symbol duration. In preferred embodiments of the invention, the frequency of the carrier symbol is downconverted to the lowest possible value and filtered.
The present invention also provides a carrier segment communications system which includes means for receiving a plurality of digital data blocks. The digital data blocks are received in a sequential manner. Furthermore, each digital data block has a discrete value associated therewith which is selected from a predetermined number of possibly assigned discrete values. In certain preferred embodiments of the invention, the predetermined number of possibly assigned discrete values is two Thus, each block of digital data is a binary digit. The system also includes means for modulating the data blocks in order to construct a carrier signal having carrier segments of predetermined shapes. The predetermined shapes are selected from a predetermined set of shapes consisting of cosine segments, sine segments, zero slope level segments, and combinations thereof. The edges of each carrier segment have a slope equal to zero. In preferred embodiments of the invention, the carrier segments are pi radians in length, so that they begin at a zero slope point and terminate at a zero slope point.
The system monitors the completion of each successive carrier segment being used to construct the carrier signal and a controller produces a control signal which indicates the correct carrier segment to be used. The controller accomplishes this task by examining the discrete value associated with the incoming digital data blocks and determining which carrier segment is representative of the digital data block being examined. It is preferred that the system further include means for transmitting the resulting carrier signal to a remote location. Furthermore, the remote location should be capable of receiving the carrier signal and demodulating it so that the base band digital signal is recovered.
When a digital signal is summed with a delayed version of itself as would happen in a multi-path condition, a complex waveform results from the destructive and constructive interaction of the amplitudes and phases that represent each digital symbol In the proposed burst mode communications technique of the present invention, simple sine waves are sampled, facilitating symbol bit edge synchronization in the preamble, and then special symbol combinations are sampled to enable optimum recognition of subsequent symbols. Special receiving techniques are then utilized to eliminate long training and equalization delays in order to maximize multi-point burst mode digital data communications.
At the beginning of the data burst or packet, determining the symbol bit edges is critical for proper synchronization. If the symbol bit edges are incorrectly detected, the entire packet can be lost, so the present invention utilizes specially created symbols for this purpose. After synchronization, the symbols must be detected with the least amount of ambiguity possible. The receiver detection process in the present invention utilizes the impaired symbol directly and does not require an equalizer to compensate for the effects of the impairment. This technique allows the synchronization and detection set up processes to occur in the first few bits of the packet.
Conventional techniques require a coherent phase reference local oscillator to phase lock onto the incoming carrier. This phase locked local oscillator is then heterodyned with the incoming data carrier to determine the phase and amplitude constellation used for demodulation. When impairments are present, equalization is necessary to keep the received signal constellation tightly grouped for optimum symbol recognition. Both ends of the communications link may even send training sequences to each other to further adapt to the communications link impairments. All of these conventional techniques require significant time delays, which reduce throughput and are very undesirable in burst mode communications systems.
In the proposed burst mode digital communications system, carrier segments and special symbols are initially transmitted at the beginning of the burst. These are designed to provide symbol edge synchronization which can be used for a long series of data symbols to follow. The amplitudes of the symbols are used as a standard from which voltage thresholds are derived. After the synchronization symbols are transmitted, the data symbols can be changed to convey the maximum amount of detectable information between digital states so as to reduce ambiguity in the detection process. This technique can be especially useful in the presence of impairments such as signal interferers, multi-path reflections, or reflections caused by non-terminated wire or cable links. When two ends of a communications link have systems based on precision high speed system clocks, the small inherent frequency differences between the two systems can be ignored if the data bursts are small or interspersed with synchronization symbols during the data bursts.
For the reception and demodulation of data symbols in the proposed system, the receiver is based on digital techniques where analog-to-digital sampling of the received waveform first takes place. These data samples are first compared against an expected pattern or template to find the symbol edge synchronization. A known transmitted sequence of special preamble symbols, digitally sampled, allows the receiver to store these samples to be used as templates, uniquely defining the pattern for each digital state Each subsequent received bit has its associated digital values compared against the stored templates to determine how many digital values matched the stored template values. The template with the highest number of matches then determines the decision as to which symbol was received. This technique is adaptive since the templates incorporate sampled digital values that already account for channel impairments. These templates are constantly updated for each data burst, which allows for increased immunity from multi-path or reflective impairments. This technique also allows very rapid acquisition of the data burst, which translates into higher data throughputs.
Accordingly, it is a principal object of the invention to provide a burst mode digital communications system which digitally creates a sinusoidal carrier by utilizing discrete carrier segments which can be added together or added with zero slope level segments.
It is another object of the invention to provide a burst mode digital communications system which digitally modulates a sinusoidal carrier using direct digital synthesis techniques to produce data symbols that are unambiguous and resistant to channel impairments.
It is yet another object of the invention to provide a burst mode digital communications system which controls the instantaneous voltage changes occurring at the bit edges without the use of filters.
It is a further object of the invention to provide a burst mode digital communications system which minimizes the amount of spurious output associated with the digital modulation of a sinusoidal carrier.
It is yet a further object of the invention to provide a burst mode digital communications system which demodulates digital data which has been modulated by carrier segments having zero slope edges.
Still another object of the invention is to provide a burst mode digital communications system which digitally demodulates the data by utilizing digital data template values captured from the incoming carrier with impairments that are compared continuously with new symbols to determine the digital symbol data.
Still a further object of the invention is to provide a burst mode digital communications system which employs a data channel with high bit rates and a narrow bandwidth that is immune to noise and signal interference.
Still a further object of the invention is to provide a burst mode digital communications system which enables multi-symbol communications using amplitude modulation in combination with frequency diversity.
It is an object of the invention to provide improved elements and arrangements thereof in a burst mode digital communications system for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes.
These and other objects of the present invention will become readily apparent upon further review of the following specification and drawings.